Room: Stars at Night Ballroom 2-3
Purpose: Proton radiography could allow for direct measurement of water-equivalent path length (WEPL) in tissue, which can then be used to determine relative stopping power (RSP). Additionally, proton radiographs allow for imaging in the beamâ€™s eye view with the particle used to deliver radiation. In this work, a proton radiography technique using a flat-panel imager and a pencil-beam scanning (PBS) system is demonstrated on phantom data.
Methods: Proton PBS plans were delivered on a Varian ProBeam system to a Perkin-Elmer XRD 1611 digital x-ray detector. Each proton plan consisted of energy layers separated by 4.8 MeV, and square field sizes at 15 cm and 25 cm. To build a calibration curve correlating detector response to WEPL, the plans were delivered to slabs of solid water while increasing thickness in 1 cm steps. Detector response is then fit as a function of WEPL pixel-by-pixel. Tissue equivalent phantoms are imaged for evaluation of WEPL accuracy and contrast-to-noise ratio. A custom-made spatial resolution phantom and an anthropomorphic body phantom are also imaged. For all experiments, the detector was run with 4x4 pixel binning, corresponding to an effective pixel size of 0.4mm x 0.4mm.
Results: Average RSP error of the imaging method, relative to reference measurements made with an IBA Zebra multi-layer ion chamber, was 0.35% with an RMSE of 1.29%. The largest error was in the low-density LN-300 lung insert (3.28%). In the spatial resolution phantom, a 2 mm deep pinhole with 1 mm diameter can be seen.
Conclusion: We have demonstrated an accurate method for measuring the WEPL in phantoms using a single flat-panel imager on a PBS system. The accuracy and spatial resolution of the method show that it could be implemented for patient position verification. Further development could lead to patient-specific measurements of RSP to be used for treatment planning.
Not Applicable / None Entered.